Crystallization and X-ray diffraction analysis of N-­terminally truncated human ALG-2

ALG-2 (apoptosis-linked gene 2) is an apoptosis-linked calcium-binding protein with five EF-hand motifs in the C-terminal region. N-­terminally truncated ALG-­2 (des3-23ALG-2) was crystallized by the vapour-diffusion method in buffer consisting of either 50 mM MES pH 6.5, 12.5%(v/v) 2-propanol and 150 mM calcium acetate or 100 mM MES pH 6.0, 15%(v/v) ethanol and 200 mM zinc acetate. Crystals of the Ca²⁺-bound form belonged to space group P2₁2₁2₁, with unit-cell parameters a = 54.8, b = 154.4, c = 237.7 Å, α = β = γ = 90°, and diffracted to 3.1 Å resolution. Crystals of the Zn²⁺-bound form belonged to space group P2₁2₁2₁, with unit-cell parameters a = 52.8, b = 147.5, c = 230.7 Å, α = β = γ = 90°, and diffracted to 3.3 Å resolution. The structures of the Ca²⁺-bound form and the Zn²⁺-bound form were solved by the molecular-replacement method. Although both crystals contained eight ALG-­2 molecules per asymmetric unit, the metal-ion locations and octameric arrangements were found to be significantly different.